Dolly Varden retire from seasonal migrations after growing to be about 12 inches in length. This reduces the risk of predation and makes for a more relaxing summer. (Photo : Morgan Bond)

Seasonal migrations are hard on fish that have to travel from their river homes to oceans and back each year. To avoid the hassle, older Alaskan trout opt for an early retirement at the point when they have grown big and strong enough to survive off of their own fat reserves, according to a University of Washington (UW) study.

"As far as we know, no one has ever seen a population of large-bodied fish come back to freshwater and just park there for the rest of their lives," Morgan Bond, lead author and a postdoctoral researcher at the National Oceanic and Atmospheric Administration (NOAA) who completed this study as UW doctoral student, said in a news release.

For their study, researchers analyzed the migratory patterns of Dolly Varden, a common and abundant trout in Southwest Alaska's rivers. To do this they used the fish's inner ear bone, or otoliths, according to the release. In total, researchers analyzed over 300 otoliths from Dolly Varden fish. This revealed a growth ring that represented each year of the fish's life and provided information regarding water chemistry, which would have differed when swimming from stream to ocean. Essentially, marine water has a higher concentration of the element strontium, so an increase in strontium on a fish's otolith signifies it migrated to the ocean that year, researchers explained in their study.

For most of the year, Dolly Varden trout live in freshwater streams. They only travel to the ocean in the summer to feed and grow. This species of trout prefers to feed on insects, crustaceans, various fish species and their eggs, as well as rodents. While the ocean is like a banquet feast compared to the fish's smaller stream habitats, the open sea also poses a series of predatory risks. Main predators of this fish include otters, bears, birds, beluga whales, seals, sea lions, and humans.

"Small fish gain enough to make this risk worthwhile because they can pack on a good deal of growth in a summer at sea," Tom Quinn, senior author of the study and a UW professor of aquatic and fishery sciences, explained in a statement.

Researchers found that when the fish have grown to about 12 inches in length they no longer need to return to the ocean each year for plentiful food sources. Instead, they retire permanently to their respective streams and rely on their unique digestive organs and a special relationship they have with their neighboring cousins - sockeye salmon - that leave behind an excess of nutritious salmon eggs each year, according to the release.

For about a month, adult Alaskan trout binge on the floating clusters of salmon eggs. During this time, the trout expand their stomachs to store the food. When spawning season ends, the fish are able to shrink their stomachs and survive for the next year on the reserves of their feast, researchers explained in their study. However, if there is a low supply of fish eggs, adult trout may have to suck it up and migrate to the ocean for food that year.